This invention relates generally to gate valves and more particularly to an improved valve seat structure for expanding gate valves. Expanding gate valves utilize expanding gate assembly structures comprising a gate element and segment which are adapted to expand transversely of one another against seating rings mounted in the valve body for effecting upstream and downstream seals.
Various types of seating rings and seating ring assemblies have been devised for such valves. With many of these, such as where the seats are press fit into seat pockets in the valve body, the seats must be installed or removed for repair purposes using special tools and associated costly procedures.
With other types of seating ring assemblies, a large drag force is developed between the gate assembly and the seating ring elements when the expanded gate assembly initiates movement from its open and closed positions. Accordingly, a very large torque force is required to move the gate assembly so as to operate the valve.
In addition, a problem always encountered with this type of valve is that of maintaining parallelism between the flat outer faces of the gate and segment elements of the gate assembly which are intended to act as sealing surfaces. When these outer faces are out of parallel, the ability of the valve to achieve a seal is impaired, particularly as regards the upstream side of the valve. While in certain installations it is only necessary for a valve to stop flow from passing out the downstream side, there are other installations where it is necessary that the valve stops flow on the upstream side to thereby block fluid in the flowline from entering the valve chamber. It is also desirable if the downstream seal develops a leak, that the upstream seal become effective. An upstream seal is also necessary to perform a block and bleed service of the valve, i.e., a service where the valve chamber can be bled to indicate if there is leakage past either seat.
It is, of course, extremely difficult to machine gate assemblies wherein the outer faces of the gate and segment elements are machined perfectly parallel and also maintained in a parallel relationship when the gate assembly is moved to its expanded condition in the open and closed positions of the valve.
It is therefore an object of the invention to provide an improved valve seat structure that compensates for out of parallelism and misalignment between the normally sealing surfaces of the gate assembly elements when the gate assembly is in the expanded condition and which is able to provide an effective seal between the gate assembly and the upstream valve seat to block flowline fluid from entering the valve chamber or to provide an effective upstream seal if leakage occurs on the downstream side of the valve.
It is another object of the invention to provide an improved seat structure for a parallel expanding gate valve which can be easily installed and removed without the use of special tools and is able to achieve sealing relationships with the gate assembly elements under out of parallel conditions of the sealing surfaces of the gate assembly.
It is still another object of the invention to provide a valve seat structure for a parallel expanding gate valve whereby the force or torque required to overcome the drag imposed by the gate assembly and to operate the valve is minimized while yet maintaining an adequate seal.
It is a further object to provide an improved expanding gate valve with a unique valve seat structure which compensates for out of parallelism between the normally sealing surfaces of the expanding gate assembly to provide an effective seal between the gate assembly and the valve seat structure when the valve is in its fully open and fully closed conditions and wherein a relatively small torque is required to drive the valve stem and operate the valve.